Read These Notes/Warnings BEFORE you Install OS X 10.9 Mavericks:
Remember OS X 10.9 is the first public release of a major OS update. I'd added some Notes and Warnings below already but many may not see/read them before clicking the news/software update links. Some reminders/warnings since many missed these warnings/notes based on posts in Apple's OS X 10.9 forum:

Also reminders below about having current backups (and preferably a bootable clone) as well as potential problems with 3rd party addons/plugins/drivers, etc - many you may have forgotten you have installed. (If you depend on the machine for daily work/business etc I'd never update early on to any OS update, especially a 'dot zero' release.)

Had a question about OS X 10.9 and ElGato EyeTV software so here's their page on Elgato software and OS X 10.9 Mavericks support that lists each of their software (EyeTV, Elgato Video Capture, Elgato Game Capture HD, Turbo.264 HD) and results of their OS X 10.9 testing so far.

For those upgrading to OS X 10.9, remember OS updates (esp. major/.0) often have problems with 3rd party addons/drivers/plugins, etc. - check the vendors site for any updates or notes on 10.9. (If in doubt, remove before updating especially any security related addons.) You can also check Apple's OS X Mavericks forum and the Desktop forums or notebook forums of your specific mac model for early feedback. As always, have a current backup. (Or 2. I also like to have a bootable clone.) Also be aware that OS X Server 2.x will not run under OS X 10.9 Mavericks. OS X Server 3.0 ($19.99 US) is out for OS X 10.9. (And per Apple's OS X server up-to-date program, if you bought a qualifying Mac mini or Mac Pro with OS X Server 2.x on or after October 1, 2013, you can download it free.)

After installing OS X 10.9 (build 13A603) on a 2012 mac mini (no 3rd party addons), SU showed iPhoto 9.5 and iMovie 10.0 updates. Later check showed iTunes 11.1.2 and iBooks Author 2.1, but not Garageband 10.0, although was a free upgrade for Mavericks. (I had never ran Garageband 6.5 that was bundled with the 2012 Mini.) FYI: I had Network Utility in the Dock but after 10.9 update it was gone (replaced with infamous "?" icon). In OS X 10.9 it's in /System/Library/CoreServices/Applications/ and also available in the System Info app under "Window" menu.

BTW: A quick check on the 2012 Mini w/OS X 10.9.0 (HDMI audio to AVR) didn't show the console message spamming (coreaudio automatic stack shots disabled/enabled with every sound played) seen since OS X 10.8.4. (I still think it was a debug option left on when compiled.)

Odd problem with Verify Boot Disk after OS X 10.9 upgrade
Not sure if anyone else has seen this but after a uneventful upgrade of OS X 10.9.0 over 10.8.5 on this Mac Mini with Intel 520 series (480GB) SSD (that has no 3rd party addons/mods/plugins), I noticed that Disk Utility "Verify Disk" (not permissions) never showed the normal (green text) message that the HD was ok. (No error messages were seen, nothing unusual in console, just seemed to finish without the OK message in disk utility.) A Verify on the 2nd (non-boot) HDD showed the normal disk is ok message. So I booted into Safe mode (shift key at boot, runs fsck and clears caches iirc) and that fixed the problem - Verify on the boot SSD now shows the usual disk OK message when it completes. (Before it didn't show any message it had completed.)

No HDMI Audio after Wake from Sleep (w/workaround):
This problem was also reported after OS X 10.8.5 update (said fixed w/Suppl. Update) and I later saw posts in Apple's forums about the problem again in OS X 10.9 (at least with HDMI). I was late being able to test for this but was able to verify this happened with my late 2012 Mac Mini (no 3rd party addons/extensions/plugins) as I'd moved it into the room with my AVR (Yamaha RX-A2000) and HDTV (Samsung) for OS X DVD Player Digital vs PCM audio tests. Long story short, after seeing the AVR display change from "PCM" (mini was set for 8ch (7.1) output) to "Analog" (losing HDMI audio) after sleep (no restore of HDMI/PCM audio on wake), instead of rebooting, what I found restored HDMI audio again was switching the Sound Prefs Output to "Built-In", putting the mac to sleep/waking it up and then switching sound prefs back to HDMI. (Just toggling the sound output prefs setting alone did not help.) But that's not really any faster than restarting. (I should have tried power cycling the AVR after wake. Didn't think of that until later.)Update: After posting the above, I decided to try again to see if turning the AVR off/back on would help but another cold start test showed no loss of HDMI audio on Wake. I waited for the AVR display to switch from "PCM" to "analog" during Mini sleep (losing mini's PCM audio) before waking. Repeated the test, no loss of audio on wake. Strange. I wonder if this may be related to 10.9's aggressive (IMO) 'energy saving'. (Earlier tonight I had twice slept and woke the mini, each time losing HDMI audio after waking from sleep.)

BTW: With OS X 10.9 the initial HDMI TV handshake does a 'double take' (2 second black screen Twice on first boot and after waking from sleep. IIRC it only happened once with 10.8.x. (But at least no HDMI dropouts seen afterwards.)

Increased Allocated Vram (Intel HD4000 example) in OS X 10.9:
Saw a mention of changes on this (not just w/HD4000, it's just the only mac I have w/onboard gpu) and checked the Late 2012 Mac Mini's onboard graphics which under OS X 10.8.5 showed 768MB memory (w/16GB RAM installed) for the Intel HD4000. In OS X 10.9, it's showing 1024MB (1GB). This mini is the only mac I have with onboard graphics (and also the only one running 10.9). (I don't use Windows on the mini but past reports noted BC/Windows still allocates more - 1.6GB IIRC.)
[Update: I later saw a Mid-2012 MacBook Pro i7 owner post that his HD4000 also shows 1024MB Vram with only 8GB of RAM installed. However an older mac owner w/HD3000 (8GB RAM also) said OS X 10.9 reports 512MB vram. (BC/Windows 8.1 reports 1.6GB vram he said.)

"OS X Mavericks v10.9 and later do not use SyncServices. Instead, Mavericks supports sharing your information using several network-based and cloud-based solutions. If you want to sync your information across multiple devices and computers, you should use one or more of these methods. These include iCloud, CardDAV servers, CalDAV servers, and servers that utilize the Exchange ActiveSync protocol."

--- Updated November 6, 2013 ---
On October 30th, 2013 Western Digital informed registered customers of affected products via E-mail regarding reports of Western Digital and other external HDD products experiencing data loss when updating to OS X Mavericks (10.9). Our investigation to date has found that for a small percentage of customers that have the WD Drive Manager, WD Raid Manager and/or WD SmartWare software applications installed on their Mac, there can be cases of a repartition and reformat of their Direct Attached Storage (DAS) devices without customer acknowledgement which can result in data loss.

WD has been tracking this issue closely through our WD Forum and through our Technical Support hotline and the occurrence rate of this event has been very low. A specific set of conditions and timing sequences between the OS and the WD software utilities has to occur to cause this issue. Should this event occur, the data on the product can likely be recovered with a third party software utility if the customer stops using the device immediately after the OS X Mavericks (10.9) upgrade. WD will be issuing updated versions of these software applications that resolve this issue.

WD strongly urges our customers to uninstall these software applications before updating to OS X Mavericks (10.9), or delay upgrading until we provide an update to the applications. If you have already upgraded to Mavericks, WD recommends that you remove these applications and restart your computer. If you have already upgraded to Mavericks and are experiencing difficulty in accessing your external hard drive, please do not save anything to the drive, disconnect the drive from your computer, and contact Western Digital Customer Service at http://support.wdc.com/contact/.

--- Updated November 5, 2013 ---
There are reports of Western Digital and other external HDD products experiencing data loss when updating to Apple's OS X Mavericks (10.9). Western Digital is urgently investigating these reports and the possible connection to the WD Drive Manager, WD Raid Manager and WD SmartWare software applications.

Until the issue is understood and the cause identified, WD strongly urges our customers to uninstall these software applications on their systems before updating to OS X Mavericks (10.9), or delay upgrading. If you have already upgraded to Mavericks, WD recommends that you remove these applications and restart your computer. WD has removed these software applications from our web site solely as a precaution as we investigate this issue.

If you have already upgraded to Mavericks and are experiencing difficulty in accessing your external hard drive, please do not save anything to the drive, disconnect the drive from your computer, and contact Western Digital Customer Service at http://support.wdc.com/country/ for further assistance.

You can now download the WD Software Uninstaller. This utility will remove Mac WD SmartWare and WD Drive Manager software. You can find the uninstaller under any of the Mac Drive Downloads sections such as the My Book Studio below.http://support.wdc.com/product/download.asp?groupid=124&sid=214&lang=en"
(Scroll down that page to see the download button. The Mac "WD Software Uninstaller" is listed as supporting Mac OS X 10.9 through 10.6.Uninstaller later removed. See Update below)

There are also some Western Digital Forum threads on problems with OS X 10.9/Mavericks. Apple's OS X 10.9 forum has a growing thread on Mavericks corrupts external hard drives that has a large number of reports, including many WD owner posts but also some from users of other brands.FYI: Some users later said setting Spotlight to NOT index external drives/volumes (add them to Spotlight Privacy list - see this post for a tip) may help, although it may not be that simple.
Update: As of Nov 25th, the WD Uninstaller is no longer at the page link in the WD post above. On Nov. 25th they also released updates to some of their WD software/utilities. I've never used them (nor do I ever plan to) but here's their forum posts on the updates: